All of the pot smokers had regularly smoked marijuana for at least five years. All of the tobacco smokers had smoked cigarettes for at least one year. Their urine tests confirmed their use of tobacco or marijuana.

Pot is illegal in New Zealand, so participants were promised total anonymity. Those who used other illegal drugs were excluded from the study.

Beasley's team wanted to find out whether long-term marijuana use was, like tobacco, associated with increased risk of emphysema. It wasn't.

However, long-term marijuana use was linked to lung problems including coughing, wheeze, chest tightness, and airflow obstruction.

Beasley's team calculates that, in terms of airflow obstruction, one marijuana joint equaled the effect of smoking 2.5 to five cigarettes at once. The finding is "of major public health significance," write the researchers.

The results are likely due to differences between tobacco cigarettes and marijuana joints, note the researchers.

"Cannabis is usually smoked without a filter and to a shorter butt length, and the smoke [has] a higher temperature," write Beasley and colleagues, adding that people inhale deeper and hold their breath longer when smoking marijuana.

The study appears in the advance online edition of the journal Thorax.